The folks at defensedistributed have a plan to end gun control as we know it. They plan to make the code to drive a 3D printer to print a single-shot .22 pistol publically available. This will, they say, make a $25 gun available to anyone with a $3000 3D printer.
Guys, the idea is swell but I seem to remember back in the '50s that a .38 round would fit into a piece of 3/8" galvanized pipe, and a nail could be driven into the primer with a rubber-band driven hammer. Tape the whole thing together to a piece of 2x4, and you were cutting edge in the back ally confrontations between the Rockets and the Jets.
Likewise a 12ga shell will fit into 3/4" pipe and the whole shebang will fit neatly enough into a piece of 1-1/4" pipe with a cap on it and a bolt through the cap for a slide-action shotgun. Harder to conceal, but works on Zombies when you've nothing else.
Neither of these rigs will cost you over $15 even at todays inflated rates, and fabrication involves little more than a hacksaw and a drill.
Modern technology has eliminated the quick and dirty .22 as the most popular material for the barrels was a chunk of car antenna, something you don't see very often any more, at least not the tubular kind.
WARNING!!: DO NOT attempt any of the above at home. Go to someone elses home first. And don't come crying to me if any of this doesn't work out the way you thought is should because I left out some inside tips or information.
Of course you don't actually have to shoot one of these. Build one, do a nice job of it, take pictures of it without yourself in the frame, and turn it in to the next gun buyback for $75-$100 or whatever they're offering.